The matching of blood flow and oxygen delivery to tissue oxygen demand is one of the most fundamental physiological processes. Recent evidence indicates that the red blood cell can act as a "sensor" and releases ATP during mismatches in oxygen demand and delivery, and this ATP can evoke vasodilation and improve local blood flow under such conditions via binding to purinergic (P2y) receptors on the endothelium. In addition to the direct vasodilatory effect, we have recently demonstrated that ATP is also capable of inhibiting sympathetic vasoconstriction ("sympatholytic"), which could further aid in blood flow and oxygen distribution. Our preliminary data indicates that the forearm vasodilator responses to ATP are not due to breakdown to adenosine, and importantly, are independent of nitric oxide and vasodilating prostaglandins. Thus, the overall goal of this exploratory research program is to directly test the hypothesis that endothelium-dependent ATP- mediated vasodilation is due to vascular smooth muscle cell hyperpolarization in humans, and to further test whether the proposed pathways are involved in vascular control in contracting muscle. To test our hypotheses we will address the following specific aims: (1) we will determine whether the forearm vasodilator responses to local intra-arterial administration of ATP are reduced by individual and combined inhibition of inward rectifying potassium channels (KIR;via barium chloride) and Na+/K+ ATPase activity (via oubain);and (2) we will determine whether the forearm vasodilator responses to graded rhythmic handgrip exercise and the ability of muscle contractions to blunt sympathetic 1-adrenergic receptor mediated vasoconstriction are impaired after inhibition of KIR channels and Na+/K+ ATPase activity in humans. The methods employed to address these aims are state-of-the-art and involve local (intra-arterial) administration of various study drugs at rest and during exercise, and measurements of forearm arterial and venous plasma ATP concentrations in young healthy humans. The findings from the proposed studies should provide unique insight into the mechanisms by which circulating ATP causes local vasodilation, and whether the hypothesized signaling pathways evoking hyperpolarization are involved in vascular control in contracting skeletal muscle. Given that impaired endothelium-dependent vasodilation is a hallmark of patients at risk or whom already exhibit cardiovascular disease, and that ATP release from red blood cells of certain patients (e.g. diabetics) is impaired, our findings regarding the mechanisms underlying ATP-mediated vasodilation could have significant implications for understanding impaired local vascular control during physiological (e.g., exercise, hypoxia) and pathophysiological (e.g., coronary and cerebrovascular ischemia) conditions in older healthy and diseased humans. PUBLIC HEALTH RELEVANCE: The studies outlined in this application are designed to address fundamental questions regarding how blood flow and oxygen delivery are controlled to peripheral tissues in humans. Understanding these basic regulatory mechanisms will provide important information that may stimulate ideas on how to improve regional blood flow and oxygen delivery in patient populations at risk for both acute and chronic cardiovascular complications.